Index-tunable photonic crystals based on ferroelectric materials provide a means for active modulation of optical signals,
and hold promises for novel device applications. In this study, (Ba,Sr)TiO<sub>3</sub> (BST)-based planar photonic crystals with
different cavity geometries were modeled. Photonic crystals with square-shaped air rod geometry, which can be prepared
in a straight-forward manner by interference lithography, were compared with photonic crystals having circular air rods.
Calculations were performed on square lattice, with either square or circular air rods, by the plane wave expansion
method. Simulation results suggested comparable bandstructures and gap maps for square or circular air rod photonic
crystal, if (1) the dimension of the air rod was small compared to the electromagnetic wavelengths inside the photonic
crystal being considered, or (2) the frequencies of the electromagnetic waves were less than 0.35(2πc/a). A better
correlation in bandstructures and gap maps between the square and circular air rod photonic crystals can be achieved, if
we compare them by the volume fractions of the photonic crystals in stead of the characteristic lengths of the rods (i.e.
diameter of the circular rod and width of the square rod).
Tunable photonic crystals (PCs) have attracted much attention in the past decade because of their various applications
such as ultra-fast optical filters and optical waveguides with add-drop functionalities. A common means of tuning PC is
by changing the refractive indices of the constituent materials via the linear or quadratic electro-optic effect, which leads
to a shift of the bandgap positions of the PC. The lead-free material, barium strontium titanate (BST), has a high
quadratic electro-optic coefficient comparable to lanthanum-modified lead zirconate titanate (PLZT), and is a promising
candidate as a lead-free tunable PC.
Here we present a study on the feasibility of developing a one-dimensional tunable PC based on a BST and magnesium
oxide (MgO) multilayer structure. The bandgap diagram of the PC structure is calculated using the plane-wave
expansion (PWE) method. For a 1% change in the refractive index of BST, a 0.99% frequency shift in the bandgap can
be achieved. It corresponds to a wavelength shift of 15.4 nm at a wavelength of 1550nm. Design of a tunable optical
filter at a wavelength of 1550nm based on a BST/MgO 1D PC is suggested. The transmission property of the 1D PC is
further verified by simulation, using the transfer matrix method (TMM).
It is of interest to the optoelectronic community if index-tunable photonic crystals can be realized by using
ferroelectric materials since the refractive index of ferroelectric materials can be electrically tuned through the
electro-optic effect. In this paper, we present our work on developing a tunable one-dimensional (1D) photonic
crystal (PC) based on a Ba<sub>0.7</sub>Sr<sub>0.3</sub>TiO<sub>3</sub>/MgO multilayer structure. A ferroelectric 1D photonic crystal consisting of a
Ba<sub>0.7</sub>Sr<sub>0.3</sub>TiO<sub>3</sub>/MgO multilayer thin film was epitaxially deposited on a MgO (001) single-crystal substrate by
pulsed laser deposition. A photonic band gap in the visible region is observed in the transmission spectrum of the
multilayer thin film. The centre wavelength of the band gap is ~ 464 nm, which agrees with the simulation result
obtained by the transfer matrix method. The band gap can be tuned by an external electric field <i>E</i>. The band gap
shifts about 2 nm under a dc voltage of 240 V (E ~ 12 MV/m).